In the production of reinforced elastomeric materials, in particular of radial tires, wire cords have been employed up to the present time, as is well known, which cords are made up of steel wires coated by electroplating with brass, preferably of a composition between Cu/Zn=60/40 and Cu/Zn=70/30 (% by weight) at the amounts between 2 and 8 g/Kg of steel and of 1-2 .mu.m thickness for a wire diameter between 0.75 and 1.40 mm. Such wires ar then wet-drawn according to a chipless procedure and employed for the production of said cords.
It it also well known that such cords, in addition to a high ultimate tensile stress and to a high flexibility, also show, during the curing process with a standard mix, an adhesion to rubber (or other elastomeric material) which is much larger than the value of adhesion that can be ascribed to the mere friction effect. At the present time such property is considered to stem substantially from the formation and growth of films of some hundred .ANG. thickness of copper and zinc sulfides at the rubber/brass interface, as a consequence of the chemical reactions occurring during vulcanization.
On the other hand, it has been observed that the adhesion between steel and the coating is also of the same importance. Thus it is clearly evident that any physical or chemical phenomenon, such as for instance corrosion, which alters the state of surfaces, affects the cord/elastomer adhesion negatively.
Such drawback manifests itself frequently, because rubber mixes commercially employed in the production of tires are hardly ever anhydrous, and give rise during vulcanization to the formation of a ZnO film, in addition to sulfides films, and next, owing to material aging, to the formation of hydroxides with the consequent definitive separation of the cord from the rubber. This was shown by a prolonged treatment with overheated steam (120.degree. C. or more), in the presence of chlorides.
In that case the examination of the state of surfaces put into evidence the presence of rust at the interface between steel and coating.
On the other hand, it is well known that car and truck tires are subjected during employment to severe mechanical stresses, especially at the highest speeds or in off-road runs.
As the safety of passengers and of load depends on tire reliability, it is necessary that the cord/elastomer adhesion be particularly strong, not only at the very beginning, but also during the full tire life.